The present invention relates in general to ultrasonic testing and electromagnetic acoustic transducers (EMATs) and, in particular, to a new and useful electrostatic shield for a coil of an electromagnetic acoustic transducer for reducing noise from various sources.
Current ultrasonic tests are contact techniques in which a piezoelectric transducer is coupled to a component surface by a fluid or gel. For electrically conductive materials, ultrasonic waves can be produced by electromagnetic acoustic wave induction. Electromagnetic acoustic transducers (EMATs) are the basis of a noncontact ultrasonic inspection method that requires no fluid couplant because the sound is produced by an electromagnetic acoustic interaction within the material. This technique can be used to eliminate the couplant, which complicates testing procedures, slows inspection rates, and can introduce errors into the measurement. In fact, in some cases, conventional ultrasonic tests cannot even be conducted because of the couplant.
In contrast to conventional contact ultrasonic testing, where a mechanical pulse is coupled to the workpiece being inspected, in an EMAT, the acoustic wave is produced by the interaction of a magnetic field with induced surface currents. The coil of the EMAT induces eddy currents at the surface of the conductor. A constant magnetic field provided by an AC, DC or pulse driven electromagnet or a permanent magnet is positioned near the coil. The interaction of the magnetic field with the induced eddy currents produces a force called the Lorentz force. This Lorentz force interacts with the material to produce an ultrasonic pulse. As shown in FIG. 1, a simple EMAT 10 consists of a coil of wire 12 and a permanent or electromagnet 14. A strong magnetic field, B, is produced at the surface of an electrically conductive workpiece 16 being tested by the permanent magnet or electromagnet 14. Eddy currents EC with density J are induced in a surface 18 of the workpiece 16 by the coil 12 which is driven at a high excitation frequency by an oscillator 20 (not shown). The Lorentz force F resulting from the alternating current flow in the presence of the magnetic field is transferred to the workpiece 16 and produces an ultrasonic wave UW (with the same frequency as the excitation frequency) that propagates through the workpiece 16.
Various configurations of the coil 12 may be used along with different directions of the magnetic field B to produce a variety of ultrasonic wave modes, with unique properties in addition to the conventional longitudinal and shear vertical (S.V.) shear waves. In conductors that are ferromagnetic, a second force (magnetostriction) is added to the Lorentz force, which makes ferromagnetic materials particularly suitable for sensitive EMAT inspection.
EMAT instrumentation involves the reception of low level signals; as such, EMATs are susceptible to noise pickup from many different sources. To minimize noise pickup, careful shielding and grounding is very important. This aspect has been recognized from the very early stages of EMAT development, and the use of shielded cables and instrumentation is well documented in the literature.
Vasile (U.S. Pat. No. 4,296,486) discloses shielded electromagnetic acoustic transducers including a source of magnetic flux (28, 30, 32, 34, 36) for establishing a static magnetic field, an electrical conductor (38) for conducting an alternating current in the static magnetic field, and an electrically conductive, nonmagnetic shield (46) disposed between the source of magnetic flux and the conductor. In the preferred embodiment, the shield (46) is provided in the form of a thin metallic sheet in contact with the source of magnetic flux and spaced from the conductor. As discussed at Col. 4, lines 3-15 of Vasile, the shield (46) acts as a ground plane and reduces losses associated with the eddy currents which are induced in the magnets by the coil (38), and the shield (46) also helps to reduce the impedance level of the EMAT (26), while causing only a minimal loss in the magnetic field strength.
Vasile thus shields his magnet from the EMAT. However, there is no known mention of shielding of the actual EMAT coil itself from the workpiece or conductor, despite the fact that the EMAT coil acts as an antenna for noise pickup from the conductor being tested as well as from electromagnetic radiation sources.
The present invention addresses this overlooked aspect and presents a unique approach to shielding EMAT coils that can provide a totally shielded EMAT system when used with the aforementioned shielded cables and instrumentation.